Re: [PATCH linux-next] mm: huge_memory: fix misused mapping_large_folio_support() for anon folios

[Zi Yan]

On 4 Jun 2024, at 0:57, David Hildenbrand wrote:

> On 04.06.24 07:47, xu.xin16@xxxxxxxxxx wrote:
>> From: Ran Xiaokai <ran.xiaokai@xxxxxxxxxx>
>>
>> When I did a large folios split test, a WARNING
>> "[ 5059.122759][  T166] Cannot split file folio to non-0 order"
>> was triggered. But my test cases are only for anonmous folios.
>> while mapping_large_folio_support() is only reasonable for page
>> cache folios.
>
> Agreed.
>
> I wonder if mapping_large_folio_support() should either
>
> a) Complain if used for anon folios, so we can detect the wrong use more easily. (VM_WARN_ON_ONCE())

This is much better.

>
> b) Return "true" for anonymous mappings, although that's more debatable.

This might fix the warning here, but the function might get wrong uses easily.

>
>>
>> In split_huge_page_to_list_to_order(), the folio passed to
>> mapping_large_folio_support() maybe anonmous folio. The
>> folio_test_anon() check is missing. So the split of the anonmous THP
>> is failed. This is also the same for shmem_mapping(). We'd better add
>> a check for both. But the shmem_mapping() in __split_huge_page() is
>> not involved, as for anonmous folios, the end parameter is set to -1, so
>> (head[i].index >= end) is always false. shmem_mapping() is not called.
>>
>> Using /sys/kernel/debug/split_huge_pages to verify this, with this
>> patch, large anon THP is successfully split and the warning is ceased.
>>
>> Signed-off-by: Ran Xiaokai <ran.xiaokai@xxxxxxxxxx>
>> Cc: xu xin <xu.xin16@xxxxxxxxxx>
>> Cc: Yang Yang <yang.yang29@xxxxxxxxxx>
>> ---
>>   mm/huge_memory.c | 38 ++++++++++++++++++++------------------
>>   1 file changed, 20 insertions(+), 18 deletions(-)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 317de2afd371..4c9c7e5ea20c 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -3009,31 +3009,33 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>>   	if (new_order >= folio_order(folio))
>>   		return -EINVAL;
>>
>> -	/* Cannot split anonymous THP to order-1 */
>> -	if (new_order == 1 && folio_test_anon(folio)) {
>> -		VM_WARN_ONCE(1, "Cannot split to order-1 folio");
>> -		return -EINVAL;
>> -	}
>> -
>>   	if (new_order) {
>>   		/* Only swapping a whole PMD-mapped folio is supported */
>>   		if (folio_test_swapcache(folio))
>>   			return -EINVAL;
>> -		/* Split shmem folio to non-zero order not supported */
>> -		if (shmem_mapping(folio->mapping)) {
>> -			VM_WARN_ONCE(1,
>> -				"Cannot split shmem folio to non-0 order");
>> -			return -EINVAL;
>> -		}
>> -		/* No split if the file system does not support large folio */
>> -		if (!mapping_large_folio_support(folio->mapping)) {
>> -			VM_WARN_ONCE(1,
>> -				"Cannot split file folio to non-0 order");
>> -			return -EINVAL;
>> +
>> +		if (folio_test_anon(folio)) {
>> +			/* Cannot split anonymous THP to order-1 */
>> +			if (new_order == 1) {
>> +				VM_WARN_ONCE(1, "Cannot split to order-1 folio");
>> +				return -EINVAL;
>> +			}
>> +		} else {
>> +			/* Split shmem folio to non-zero order not supported */
>> +			if (shmem_mapping(folio->mapping)) {
>> +				VM_WARN_ONCE(1,
>> +					"Cannot split shmem folio to non-0 order");
>> +				return -EINVAL;
>> +			}
>> +			/* No split if the file system does not support large folio */
>> +			if (!mapping_large_folio_support(folio->mapping)) {
>> +				VM_WARN_ONCE(1,
>> +					"Cannot split file folio to non-0 order");
>> +				return -EINVAL;
>> +			}
>>   		}
>>   	}
>
> What about the following sequence:
>
> if (folio_test_anon(folio)) {
> 	if (new_order == 1)
> 		...
> } else if (new_order) {
> 	if (shmem_mapping(...))
> 		...
> 	...
> }
>
> if (folio_test_swapcache(folio) && new_order)
> 	return -EINVAL;
>
> Should result in less churn and reduce indentation level.

Yeah, this looks better to me.

Best Regards,
Yan, Zi

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